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Committed development & traffic data

The latest observed base transport data (traffic counts, passenger counts) should be used. Data should be up-to-date and relevant and should be agreed with TfL and other relevant authorities to ensure surveys do not clash with unusual events or works that may impact on the network of interest

In most cases, TfL expects traffic surveys to be undertaken during school term times, a neutral month and neutral weekday; the observed local highway peak should form the basis of your TA. It is good practice to use validation data when collecting traffic survey data

For certain land uses (such as a tourist site, sports venue, or place of worship) or due to other aspects, use of surveys at specific times maybe more appropriate

The use of historic data (data older than two years) will generally not be accepted unless there is a valid reason (such as long-term road works) and that it is verified by recent observed data. TfL will generally expect TA data and associated models to be updated at the planning stage as well as at Traffic Management Act Notification (TMAN) stage

Traffic growth & committed development

Growth assumptions for future years should be agreed with TfL and based on observed trends from approved data sources nearest to the site. The level of detail will generally depend on the scale of development and if the wider area is subject to significant change in quantum, land use and traffic conditions

TfL recommends either the application of generic growth rates such as Trip End Model Presentation Program (Tempro), extraction of data from demand or assignment models managed by TfL or others, or site-by-site assessment in the area of influence

Identification of up-to-date and relevant committed development sites must be provided by the local planning authorities or development agencies and reviewed by TfL. Assumptions about trip generation from these sites should be presented to TfL to check and confirm at an early stage

Where a site is vacant or partially vacant, the quantification of trips (and modal distribution) should be based upon the permitted uses. However, TfL will normally request sensitivity testing to gauge the impact of the development against the measured number of trips that the existing site is generating at the time of the application - in order to ensure that the transport impact of the development can be quantified relative to existing conditions

For the purposes of the TA, only committed transport schemes should be considered. TfL will also request sensitivity testing to take account of potential cumulative impact if there are concurrent planning applications deemed to represent a significant trip generator/attractor, even if these known schemes are yet to be determined

Transport investment in London is primarily identified in the TfL Business Plan, the London Plan and other specific documents prepared by TfL. Information provided by local planning authorities, Highways Agency, Network Rail, the Government and others are further sources of potential transport investment information. TfL agreement should be obtained on how to take account of these plans in the assessment of trip generation and base transport loading

Trip generation

The development quantum and land use being assessed needs to be clearly identified and quantified in the TA report, including how the development is to be constructed, phased and occupied

For most land uses TfL expects trip generation to be based on person trips per 100 square metres. This should be set out in the TA. Exceptions will need to be explained in the TA and preferably agreed in advance with TfL

For design purposes, assumptions about employee density (such as one employee per 16 square metres gross floor area for office) or residential occupancy (for example, average household size) or known staffing, student or pupil numbers can be used to verify person trip rates. If this approach is being adopted TfL would need to agree the assumptions are reasonable and confirm the relationship with peak hour generation (eg can a normal distribution be assumed? Is the upper bound limited by the planning permission being sought?)

For residential uses, trip rates are usually based on rate per residential unit or household. If there is a balanced mix of unit sizes then a single rate for the site maybe acceptable or a rate per bedroom applied. Account should be taken of the fact that larger units will generate more trips than smaller units

Housing tenure and type may also impact on trip rates as would specialist housing for specific groups eg student housing or retirement flats, and the choice of surveys used for trip generation should reflect this

Where sites are currently in use, TfL would expect to see surveys to ascertain current levels of trip generation rather than relying on data from alternative sources such as the trip generation database TRICS

When using TRICS, sites more than five years old must be excluded unless otherwise agreed with TfL. The sites used should have comparable characteristics including use, scale, PTAL and car parking. The criteria used in selecting sites should be clearly stated and agreed by TfL in advance of the TA submission

Data collected by private organisations can also be used but only if TfL has verified the methodology and validity of the data before accepting it is valid for TA purposes

TfL seeks to assess a standard 'busy hour' - usually morning and evening peaks, and often Saturday or Sunday peak and inter-peak, in particular for retail or leisure uses where it is known that congestion is particularly high at the weekend. This may vary by land use, mode of travel and location, and hours of assessment should be agreed prior to further work on trip generation being carried out

As well as person trips, TfL expects to see delivery and service vehicle trips assessed by numbers and type as well coaches. The site will need to be designed to accommodate regular large vehicles (e.g. refuse and removal vans) but also infrequent trips. The TA should provide evidence to quantify demand

Construction impact is an increasing concern in London. The TA should assess the number of heavy goods vehicles and the amount of general construction traffic generated during construction including construction workers phasing and any restriction caused by construction traffic or activity. For more information on how this should be done and how the impact of freight movements can mitigated, please see the Deliveries in London section

Mode share

The data sources above can be used to determine mode share. In most case this will suffice subject to TfL agreeing the suitability of selected sites. Councils also hold and monitor travel planning data using systems such as i-Trace which can also be used to determine mode share at similar sites

The use of data published by TfL (eg Travel in London report, London Travel Demand Survey) or government (eg census) can be used, subject to use of the latest published data and clear identification of the data set used

The use of census data to assess mode of travel to or from a specific area may be appropriate as it includes the entire population on a given day. However, it is limited as the data collected relates to travel to work and does not relate to time of travel

Many trips in London are multi-modal. The main mode is the mode used for the longest part of the journey. Mode of arrival or departure from a specific site may be the critical mode that needs to be assessed (eg where residents catch a bus to a more distant rail station)

Assumptions about mode of travel should be checked against the proximity of the mode to site, frequency and ease of access. Congestion and crowding may also impact on mode choice and initial assumptions about the future transport network should be agreed with TfL

Walking, bus and cycle are often used as access modes for journeys less than two miles and may be under-reported in databases. TfL expects the TA to take account of this aspect

Car parking supply can impact on mode share. Available off-site car parking capacity should be measured and TfL should agree the impacts this may have on mode share

For residential land uses, there is a correlation between car ownership, car parking and car use that is influenced by local context and site specific factors. This should be examined in the TA

Distribution of trips to transport network

In most cases, this can be assessed with reference to existing local travel demand (eg bus loadings or traffic turning proportions)

In some cases an assignment model will be provided for review. This could be a simple desktop or more complex spreadsheet-based model. TfL would need to be able to check the assumptions in detail and data used

Specific software can be used to assign traffic or passengers to the wider network (eg SATURN or Highway Assignment Models aka HAM, Railplan for public transport). Use of these in the TA as well as access to TfL maintained transport models will need to be discussed and agreed with TfL as part of the formal pre-application advice service

For assignment of trips to the public transport network, the TA needs to provide information on the likely origin or destination for each public transport mode. This could simply be bi-directional, or could relate to many points within the area of interest. The assignment of trips to particular services will be assessed by TfL specialists. Further detail can be found in the Guidance by transport type section

Passby, diverted, or linked trips

As a starting point for assessment, TAs should assume person trips will be new to the transport network.

Where a TA assumes passby, diverted or linked trips, the TA must provide justification for specific assumptions with a transparent approach so TfL can check assumptions at each point on the network.

For a large site with multiple uses TfL can provide advice on linked trips between land uses. Within large mixed-use schemes, certain uses will not generate additional trips apart from walk and cycle

Different assumptions could apply for different time periods. For example, the proportion of passby trips into a retail development will generally be lower at weekends than during the weekday peak

Where passengers board or alight public transport closest to the site, new trips should be assumed. TfL will consider how this impacts on loading

Where turning on or off the highway network, new trips should be assumed. The TA needs to justify if subtracting from link flow or adding to link flow